PROJECT SUMMARY Of the 10 million people infected with the Human T-cell Leukemia Virus type 1 (HTLV-1) retrovirus, about 5% will develop a fatal leukemia/lymphoma, and up to 3% will develop a progressive neurodegenerative disorder. These diseases arise from one or a few infected T-cells among many thousands becoming pathogenic by first evolving survival and proliferation advantages. These properties are believed to be acquired through a series of interconnected events following the initial infection. Consistent with these overall observations, the probability of a newly infected T-cell ultimately becoming pathogenic is low. Therefore, the efficiency of HTLV-1 spread within the host plays an important role in determining whether the infection will lead to disease. Among T-cells, HTLV-1 infection occurs through direct contact between infected and target cells. These interactions are initiated and stabilized by intercellular adhesion molecule 1 (ICAM-1) on the infected cell engaging with lymphocyte function-associated antigen 1 on the target cell. This intercellular adhesion event triggers formation of a supramolecular complex at the cell-cell junction known as a virological synapse (VS). Viral particles accumulate at the VS, which provides a specialized microenvironment for these particles to transit to the target cell. One viral protein, Tax, has been found to play a central role in VS formation and infection. Preliminary data in this application reveal that a second viral protein known as HBZ enhances HTLV-1 infection, which is correlated with its ability to activate expression of ICAM-1 and neuropilin-1 (Nrp-1). Nrp-1 is one of the HTLV-1 receptors; however, in the context of the infected cell, its role has not yet been characterized. Interestingly, the functions of Nrp-1 are precisely adapted for retention of viral particles on the infected cell-surface and translocation of particles to the VS. This application is based on the hypothesis that HBZ enhances HTLV-1 infection by activating ICAM-1 and Nrp-1 expression. Specific Aim 1 is to characterize the contribution of HBZ to infection. First, we will examine whether HBZ enhances infection of primary human CD4+ T-cell and dendritic cells. These cell-types are essential for HTLV-1 spread within the host. Second, we will examine a Tax- independent role of HBZ in promoting infection, which would explain how de novo infection persists in the host when Tax expression is low or absent. Specific Aim 2 is to analyze the contribution of ICAM-1 and Nrp-1 to HTLV-1 infection. We will test whether an increase in ICAM-1 expression on the surface of the infected T-cell is a requirement for infection, which has not been reported previously. We will also address the potential role of Nrp-1 in VS formation and in initiating/stabilizing infected T-cell/dendritic cell interactions. Specific Aim 3 is to characterize the mechanisms used by HBZ to activate ICAM-1 and Nrp-1 expression. Given that HBZ is a transcriptional regulator, we will focus on how the viral protein affects the level of ICAM1 and NRP1 gene transcription. Results from these aims will define novel processes involved in HTLV-1 infection, which is essential for the broader goal of designing therapeutic approaches that prevent HTLV-1 spread within the host.